This year’s fall rains and clouds following a summer drought could cause nitrate poisoning of livestock – from improved forages or from weeds, says Vanessa Corriher, Texas AgriLife Extension Service forage specialist.

“In a recent incident, a Sabine County producer turned some cattle into a dry lot,” she says. “Although he supplied hay, the cattle apparently died of nitrate poisoning from eating pigweed in the lot.”

Livestock generally won’t consume weeds when quality hay is available, she says, but in this instance they did and several cows died.

Forages and small grains such as sudangrass, sorghum-sudan hybrids, pearl millet, corn, wheat and oats can build up high levels of nitrates. Canada thistle, pigweed, smartweed, ragweed, lambsquarter, goldenrod, nightshades, bindweed, Russian thistle and stinging nettle are some weeds that also produce higher amounts of nitrates.

Hay cut during or just after a drought, especially if nitrogen was applied just before harvest, also adds risk, Corriher says.

Nitrates are present in all forages. Once nitrate levels are built up in hay, they stay there; hay harvested with high levels of nitrates will still be hazardous months later. “Though the risk of nitrate poisoning is higher after a drought or an extended period of cool, wet weather, it’s something producers should be aware of year-round,” she warns.

With normal levels of nitrates, a range animal’s rumen converts nitrates into nitrite, which converts to ammonia, then to amino acids and, finally, to proteins. But when nitrate levels are high in forages, large amounts of nitrites are absorbed directly into the bloodstream through the rumen wall. Nitrites convert hemoglobin in the blood into a form that can’t transport oxygen. The blood turns from a bright red to a chocolate color, and the animal essentially dies of asphyxiation, Corriher says.

Producers should regularly take samples of forages and weeds, at various growth stages, and have them analyzed for nitrates. “Be sure to specify that you want nitrate analysis,” she says. “Standard nutritional analysis usually does not test for nitrates.”

Instructions and sample submittal forms can be found on the AgriLife Extension Soil, Water and Forage Testing Laboratory Web site. Or call 979-845-4816.The Soil, Plant, Water Analysis laboratory at Stephen F. Austin State University in Nacogdoches also does forage analysis. Contact the lab at 936-468-4500 or
Fact sheets on nitrate and prussic acid poisoning can be found online at the AgriLife Bookstore. Search for documents E-543 and L-5231.